1. Field of the Invention
This invention relates to methods and systems for laser soft marking, especially for semiconductor wafers and devices.
2. Background Art
Lasers have been used for laser marking semiconductor wafers for decades. A listing of representative patents and publications generally related to laser marking is now provided. U.S. Pat. No. 5,329,090 relates to dot marking of wafers. The following representative patent references relate to various aspects of laser marking of wafers and electronic assemblies, illumination, and inspection/reading marks: U.S. Pat. Nos. 4,522,656; 4,945,204; 6,309,943; 6,262,388; 5,929,997; 5,690,846; 5,894,530; 5,737,122; and Japanese Patent Abstract 11135390.
The following representative references provide general information on various laser marking methods and system configurations and components: “Galvanometric and Resonant Low Inertia Scanners”, Montagu, in Laser Beam Scanning, Marcel-Dekker, 1985, pp. 214-216; “Marking Applications now Encompass Many Materials”, Hayes, in Laser Focus World, February 1997, pp. 153-160; “Commercial Fiber Lasers Take on Industrial Markets”, Laser Focus World, May 1997, pp. 143-150. Patent Publications: WO 96/16767, WO 98/53949, U.S. Pat. Nos. 5,965,042; 5,942,137; 5,932,119; 5,719,372; 5,635,976; 5,600,478; 5,521,628; 5,357,077; 4,985,780; 4,945,204; 4,922,077; 4,758,848; 4,734,558; 4,856,053; 4,323,755; 4,220,842; 4,156,124.
Published Patent Applications WO 0154854, publication date Aug. 2, 2001, entitled “Laser Scanning Method and System for Marking Articles such as Printed Circuit Boards, Integrated Circuits, and the Like” and WO 0161275, published on Aug. 23, 2001, entitled “Method and System for Automatically Generating Reference Height Data for use in a Three-Dimensional Inspection System” are both assigned to the assignee of the present invention. Both applications are hereby incorporated by reference in their entirety.
The visibility of laser marks as seen by a vision system (or by operator visual inspection) may depend on several factors including mark depth, debris, etc. which in turn depend on laser material-interaction. For certain wafer marking applications the conventional wisdom leads to relatively large marking depths which may provide for good readability, but increasing susceptibility to subsurface damage.
Wafer marking systems have long been provided by the assignee of the present invention. WaferMark™ system, produced by the assignee of the present invention for several years, is believed to be the first industrial laser marking system on silicon wafer. Specifications include a 120 μm marking dot diameter hard marking for 300 nm wafers. This meets the SEMI standard specification M1.15. A “soft marking specification” exists for wafer back side soft marking, including marking rough surface back side wafers up to 200 mm wafer. On the “Sigma Clean” system, a backside-marking option is provided for both front and backside marking for up to 200 mm wafer.
There are roughly two kinds of laser marks currently used by the industry, namely soft marks and hard marks. Various marking systems for producing both “hard marks” and “soft marks” are manufactured by the assignee of the present invention. One such currently available system is the GSILumonics Wafermark® Sigma Clean® is used to produce a type of softmark called Supersoftmark®. This mark is generally characterized as “debris free”. These marks are typically produced with diode pumped, q-switched pulse laser systems. Such Supersoftmarks® are produced with the laser system typically operating in a narrow “energy window”, a range of energies having an upper limit and lower limit wherein acceptable marking occurs.
U.S. Pat. No. 4,522,656, assigned to the assignee of the present invention, is the foundation of the current laser technology for soft marks. It describes a method which is characterized by the steps of irradiating by means of a laser pulse, a surface segment which has a surface area corresponding to 1.5 times to 6.5 times the surface area of the desired surface pattern, and adjusting the energy of the laser pulse so that only in the center of the surface segment, and on a surface corresponding to the surface pattern, the semiconductor material is melted and partially vaporized. The pattern generated usually has, relative to the original semiconductor surface, a raised annular rim and a recessed center, as shown in FIG. 5.
According to U.S. Pat. No. 4,522,656, the depth of the recess can be controlled by energy density, i.e., the depth of the recess will be increased if there is a corresponding increase of the energy density in the center of the pulse.
It is well known, however, that the process window for the energy density for generating such marks (commonly known as the super soft marks) is very small. Therefore, the adjustment of the depth of the recessed area by changing the energy density on the part is very limited. In addition, there are certain depth ranges that cannot be achieved by simply adjusting the energy or energy density.
For example, one can increase the laser energy from 970 μj to 980 μj to get the upper energy limit of the super soft mark, shown in FIG. 6. Since the energy process window for super soft mark in only around 10 μj in this case, the difference in depth between the two marks is very small. If one has to generate a mark with a depth about 2 μm, for example, it is very difficult, and perhaps impossible, with of laser marking technology developed to date.
As the semiconductor fabrication technologies evolve, soft marks on wafers with different mark depths are required to accommodate the new processes. It is, therefore, very desirable to have a laser marking technique that provides easy adjustment for the marking depth of the super soft marks.